Hobbing – introduction
The simple way of cutting gears is by using the Browne and Sharpe type of cutters.
fig a typical Brown and Sharpe type gear cutter
The problem with these is, that for a given size of tooth, a set of eight cutters is required to make a gear with any number of teeth. Furthermore the cutting of each tooth (i.e. each gap) is a separate operation requiring intervention from the operator. Not only is each intervention time consuming but it is an excellent opportunity for something to go wrong.
Gears can be cut more efficiently by “hobbing”.
Hobbing is done using a cutter known as a hob. Any one hob can only cut one size of tooth but it can cut a gear with any number of teeth.
fig a hob
The hob is essential a screw thread that is slit lengthwise to produce teeth. Each tooth has to be releived. If the shape of the teeth is that like an ACME thread, ie the sides are straight lines at an angle, then this will cut an involute type of gear tooth. The angle of the sides of the teeth on the hob will be the pressure angle of the teeth on the gear being cut.
That’s the easy bit. The tricky bit is that the hob is rotated and at the same time the workpiece is slowly past over it and is rotated at the same time. The relationship between the rate of rotation of the hob and the workpiece is critical. At the same time, the rate of rotation of the workpiece and the rate at which it is feed over the hob is critical.
A further problem is that the teeth on the hob are at angle to its axis. Consequently, to produce straight gears, the workpiece must cross the hob at this same angle.
One way of doing this is by using a machine especially designed to do this. A popular one is that based on a design by Mr Jacobs. The original of this is in the SMEE museum. It is also possible to buy the castings necessary to make it from CES.
This machine in its simplest form only produces spur gears, ie, gears with straight teeth.
This machine can be modified to produce helical gears. One example of this is the machine made by Giles Parks.
On close examination it will be found that most of the hardware in the gear hobber already exists in a vertical milling machine fitted with a dividing head. However to be turned into a hobber there needs to be a linkage between the vertical spindle and the milling table etc. Doing this by mechanical means is almost impossible to do so that the resulting hobber is flexible enough to make it usable. It is probably possible if the linkage was done electronically. That is there is a shaft encoder at the top end if the spindle. The output of this is divided electronically and the output of the divider is used to drive the dividing head. The output of this is divided again and is used to drive the milling table.
Mechanical gearing for hobbing
During the course of hobbing a gear the hob might go round several hundred times. Any error will be added each time so the errors will accumulate over the time of cutting the whole gear. This means the accuracy of the gear ratios is very important. This cannot be done using the set of gears that is usually provided with a dividing head.
Either each of the gears trains will need six gears rather than four. Or a proper set of gears will be needed. That is, most of the gears between twenty and one hundred and twenty. These can, of course, be made using the gear hobber itself.
Gear ratios can be computed using Brocots method.
Or by use of software such a Hobnail